65 Monza 140 - Slight miss at steady throttle

[rammer123]

Well, after some great input from Brad Bodie I had some direction on how to proceed with troubleshooting my performance issue. I was unable to get the secondary's to close all the way (suspected vacuum leak). So, I taped off the bores so they couldn't suck any air at idle when I did a manifold vacuum test.

Vacuum bounces wildly between 12 hg and 20 hg at idle on the manifold vacuum on both sides. If I accelerate to 1500 rpm it settles in and reads between 17-18 hg still bouncing slightly both sides. What should I see for a normal vacuum reading?

Last Fall I had a bad fuel leak at one of the fuel line connections which started this whole fiasco. I had carbs rebuilt and replaced fuel lines. When I restarted this spring with rebuilds done I had a lot of smoke coming from the side the fuel leaked out on. I wasn't sure if fuel had entered the muffler on that side and it was burning out or fuel mixed with oil on that side of the engine and was burning off or what. Couldn't really tell. After I ran the car at idle in my driveway 5-6 times and drove it trying to sync carbs , it finally stopped smoking. The car has not run right since all this started. Starts and accelerates fine, but has a slight miss at steady throttle.

Also, carbs have been rebuilt again, by me. New spark plugs, wires cap, rotor and points as well.

[rowin4]

Do a compression check , that might be the reason for the vacuum is not stable.

[flat6_musik]

So, I taped off the bores so they couldn't suck any air at idle when I did a manifold vacuum test.

I hope you taped those carbs off BELOW the throttle plates, not up on top near the air horn/choke plate.

[rammer123]

Why not on top? Where else can air get sucked in on secondary's if not down the bore? Also, do you know what a good vacuum reading should be at idle?

[bbodie52]

So, I taped off the bores so they couldn't suck any air at idle when I did a manifold vacuum test.

I hope you taped those carbs off BELOW the throttle plates, not up on top near the air horn/choke plate.

Why not on top? Where else can air get sucked in on secondary's if not down the bore? Also, do you know what a good vacuum reading should be at idle?

Carburetors accomplish their work with fuel control on the basis of vacuum and airflow. If you close off the secondary carburetor air intake at the top and there is a known problem with a vacuum leak at the throttle butterfly valves at the base of each carburetor throat, closing off the throat at the top would create a vacuum chamber between the top and bottom of the carburetor throat. With normal air pressure in the float bowls, a vacuum in the carburetor throat would tend to draw fuel from the float bowl through the mixture passage and discharge nozzle and into the carburetor throat. You would have a raw fuel buildup drawn into the intake manifolds through the partially closed secondary throttle butterfly valves. The initial problem was apparently an air vacuum leak into the intake manifolds, via the secondaries. Capping the top of the secondary will change the nature of the leak, by adding fuel to the mix and slowing the airflow. The outcome would change the problem for the engine, but would not eliminate it. Capping the intake manifolds below the secondaries would stop the air leak completely, instead of allowing it to continue while drawing fuel into the mix.

[66CorsaConv]

My 66 Convertible 140 had a light throttle sputter of farting going on from the day I picked it up. On certain roads from the dual exhaust I could hear the intermittant miss/fart/sputter and a disturbance in the otherwise smoothe power flow. I suspected vacuum leaks causing bad low and part throttle air fuel control.

After a few summers the tachometer became flaky too. The needle would be jumpy at the same part throttle point. And any time I got into the secondaries lots of knocking and pinging. Even with fresh premium. We only put about 250 miles on it per year so I lived with it. Then got fed up.

So when I put it to bed Fall 2013 I pulled the primary carbs and the distributor bringing them home to the shop for some TLC over the winter. The guys here said that when rebuilding the carbs new $10 throttle shafts were a must.

They were sooooooooooooo right! My shafts were worn almost 0.010" and quite loose.

I also remembered little ground wires inside some Corvair distributors way back when. So I bought a rebuilt upgrade Clark's distributor breaker plate with ground strap wire.

With Brad's advice I made some other corrections to the distributor besides making sure vacuum advance and the springs and weights were good and clean and operating properly. I will probably get some ribbing since I cleaned the points, rotor, cap and re-used them. I'm cheap!

My slight miss was twofold. Leaks at worn throttle shafts and a well worn distributor breaker plate that had grounding issues. The old breaker plate had about 0.020" of slop in the factory riveted pivot point.

I'm not 16 anymore and I did not expect to want the secondaries on par with the primaries. Well I'm reconsidering that. This thing is fun to romp on. The miss, the knock, and jumpy tach is GONE, tach is rock solid!

Brad is right on the "sealing off" of secondaries. I would only use block off plates and bolt them where the carbs go. There are enough leaks past even a fully closed throttle blade and especially the idle transfer slots to get vacuum up to the main venturi cluster and draw unwanted fuel.

[rammer123]

Thanks for all the advice guys. I will remove secondary's completely and block the intakes and recheck my vacuum readings. I do know the throttle shafts are new. Also, will check all the parts in distributor advance, springs , weights breaker plate. Still looking for an answer on what a good steady vacuum reading should be at idle??

[66CorsaConv]

I'm not sure on what a "good" vacuum is on the Corvair 140 since this one is my first one. I would imagine more than 12" and I doubt you will get 20" Steady.

The bouncing wildly is of concern. You do have the carburetor balance tube still hooked up? Good rubber connections to it? Sometimes there is a vacuum port on the balance tube. I believe Power Glides need that for the shift modulator.

Brad has posted all sorts of pictures of these things.

A not steady vacuum to me has usually been valves that are not sealing or some other pumping problem. As another post said, a compression check may be in order.

When I'm doing compression checks I usually put shop air down the spark plug. I have a set of adapters for this but they are newer plug taper seat type. So I also have been known to remove the Schrader Valve from my flat gasket Corvair style plug, Snap On adapter. The Snap On quick connect matches my shop air quick connects.

When you put air into a cylinder near TDC firing the engine will usually move to Bottom Dead Center valves closed due the air pressure. Any bent, burned valves will hiss and leak. Air will come out of a carburetor with a bent or not closing intake valve. Air will come out of a bent or burned exhaust.

A blown head gasket on a Corvair is uaually easy to hear.

Hope you find it.

[flat6_musik]

Still looking for an answer on what a good steady vacuum reading should be at idle??

I would say, in neutral, about 15"~17". The more you advance the timing, the higher the vacuum will go, to a point......but then, of course, detonation comes into play when you accelerate. A steady vacuum reading is what you want, even if it's a bit on the low side, say 13~14 inches. A motor with weak compression will tend to give you low vacuum, IIRC.

[rammer123]

Thanks for the answer about "steady vacuum reading". I appreciate everyone's advice. I will continue this discussion when I have new findings.